The present invention is concerned with a vacuum brake force booster comprising a mobile wall sealed against the booster casing and acting upon a control housing. A piston rod is coupled to a brake pedal for operating a dual valve permitting the selective connection of a work chamber of the brake force booster to vacuum or to a higher differential pressure. The dual valve is provided with a valve piston and an intermediate piston axially displaceable in the control housing and has a master cylinder connected to the booster casing for two brake circuits.
In known brake systems such as shown in German Pat. No. 1182539, there is a vacuum booster especially used for operating the brakes of automotive vehicles. The control housing connected to the booster piston and a power transmission element for the master cylinder are movable with respect to one another. Means are provided between the control housing and the power transmission element to transfer the force of the booster piston to the power transmission element and the reactive force to the operator through the control member of the valve means. As a result, a sealed chamber filled with fluid will be formed by the control housing and the power transmission element, with a means being provided on which the pressure prevailing in that chamber is able to exert a reactive force to the valve piston of the valve means.
Such a conventional brake force booster has the disadvantage that the fluid contained in the sealed space cannot be refilled without dissembling the booster in the event that part of the fluid has escaped.